One very obvious difference between a religion and mysticism/spirituality/ritualism/cultural practice/etc is that, once the Bible's canon was decided on, then that was it. It didn't change anymore, you can't discuss it, and if you changed it (through translation or addition of other books) then you had to start a whole new sect of the religion and break away from the church. There's centralization around a head, canonization of specific books, strict definition, etc. Dogma.

It's that process, where a spiritual belief system crystalizes around a centralizing dogmatic power structure, that it becomes a religion. There's a reason Eastern traditions of Christianity had less of an interest in defining a strict religious canon as opposed to Western traditions (though as the West took over the world it imposed religion on everyone, from Hindus to Buddhists to Orthodox Christians). It's modernist, and actually tied quite strongly to the printing press.